In general, an artificial (prosthetic) knee joint will be prescribed for a person with a through-knee (TK) or an above-knee (AK) amputation, i.e. a person without a knee joint, shank or foot. The ability for the knee to bend or articulate allows for activities such as sitting; it also allows the leg to swing during the swing-phase of gait (walking or running).
When standing or putting weight on the leg, as during the support-phase or stance-phase of the gait cycle it is undesirable for the prosthetic knee to bend uncontrollably as this will cause the amputee to fail. This is referred to as “stance-phase control”. Amputees have some control during stance by the way they load the leg and how they use their remaining muscles at the hip. Alternatively, a prosthetist can align a prosthesis to be more or less stable by placing the knee joint axis behind the load bearing plane or load line. However, this tends not to produce ideal gait characteristics.
While many different designs have been proposed, the majority of prosthetic knee joints are designed to address the issue of stance-phase control, i.e. keeping the knee from articulating when the prosthesis is supposed to be providing support. A prosthetic knee joint may have a built-in “locking” mechanism for this purpose.
One type of lock is weight activated and provides the two conditions for when the knee is to be locked and when it should bend freely; that is, during weight-bearing and non-weight bearing respectively. However, this is not ideal because, for a natural gait and normal initiation of the swing-phase, the knee should begin to flex at the end of the stance-phase, even as the leg is still under load. It can be appreciated that this is not possible with a weight-activated knee since it remains locked as long as the prosthesis is under load.
A supplementary condition is needed in the control of the knee lock, so that during weight-bearing, the knee lock is inactive when the forefoot is loaded, or similarly, the knee lock is activated only when the rear of the foot is loaded. This is described in patents U.S. Pat. No. 3,015,825 and U.S. Pat. No. 5,704,945 and by C. W. Radcliffe (Bulletin of Prosthetics Research—Fall 1977 and—Spring 1979).